Phestia rostellata from the Arkona formation of Ontario

Among the tiny pyritized shells from the Arkona formation is the Pelecypod Nuculana Phestia rostellata. These seem to be internal molds rather than replacement of shell material. I've only found one but others that I've seen seem to match this theory. Edit 5/31/15: New information has led me to rename this and similar specimens from the Arkona shale to Phestia rostellata as Nuculana is an invalid genus nomen for Paleozoic species of this type of pelecypod.

There is something about Paleozoic Pelecypods and the lack of preservation of their shells.